The invention relates to metal/air battery constructions and to applications thereof.
Metal/air batteries produce electricity by the electrochemical coupling of a reactive metallic anode to an air cathode through a suitable electrolyte in a cell. As is well known in the art, an air cathode is a typically sheetlike member, having opposite surfaces respectively exposed to the atmosphere and to the aqueous electrolyte of the cell, in which (during cell operation) atmospheric oxygen dissociates while metal of the anode oxidizes, providing a usable electric current flow through external circuitry connected between the anode and cathode. The air cathode must be permeable to air but substantially hydrophobic (so that aqueous electrolyte will not seep or leak through it), and must incorporate an electrically conductive element to which the external circuitry can be connected; for instance, in present-day commercial practice, the air cathode is commonly constituted of active carbon (with or without an added dissociation-promoting catalyst) containing a finely divided hydrophobic polymeric material and incorporating a metal screen as the conductive element. A variety of anode metals have been used or proposed; among them, alloys of aluminium and alloys of magnesium are considered especially advantageous for particular applications, owing to their low cost, light weight, and ability to function as anodes in metal/air batteries using neutral electrolytes such as sea water or other aqueous saline solutions.
Thus, by way of more specific example, an illustrative aluminum/air cell comprises a body of aqueous saline electrolyte, a sheetlike air cathode having one surface exposed to the electrolyte and the other surface exposed to air, and an aluminum alloy anode member (e.g. a flat plate) immersed in the electrolyte in facing spaced relation to the firstmentioned cathode surface. The discharge reaction for this cell may be written EQU 4Al+30.sub.2 +6H.sub.2 O.fwdarw.4Al(OH).sub.3.
As the reaction proceeds, copious production of the aluminum hydroxide reaction product (initially having a gel-like consistency) in the space between anode and cathode ultimately interferes with cell operation, necessitating periodic cleaning and electrolyte replacement. Recharging of the cell is effected mechanically, by replacing the aluminum anode when substantial anode metal has been consumed in the cell reaction.
Metal/air batteries have an essentially infinite shelf-storage life so long as they are not activated with electrolyte, making them very suitable for standby or emergency uses. For example, an emergency lamp or lantern can be constructed with a metal/air battery such as an aluminum/air battery, and a separate container of electrolyte can be stored with the battery, or be readily available within its intended environment of use. When a need for use of an emergency light arises, a user can merely activate the metal-air battery (by immersing the electrode in the electrolyte) and be provided with useful light.
As any consumer can appreciate, a lantern with an infinite storage life is much more reliable than common dry-cell battery-powered lanterns, having batteries which tend to deteriorate with shelf storage. Reaching for a dry-cell-powered lantern in an emergency, only to find that the batteries have deteriorated to a discharged condition, is a frustration experienced by many people. A metal/air-battery-powered lantern avoids such a problem, because the cells cannot be depleted until the battery is filled with electrolyte.
The voltage of a single metal/air cell such as a magnesium or aluminum air cell is or may be less than that required for a lantern or other use. In such case, as well as for other purposes, a plurality (typically two) of the cells may be connected in series. Desirable characteristics of a plural-cell metal/air battery include structural simplicity and compactness, ease of activation (bringing the electrodes into contact with electrolyte) by an unskilled user, and avoidance of current paths through the electrolyte between electrodes of like polarity in different cells.
The provision of a metal/air battery-powered lantern for emergency situations is proposed in Watakabe, "Magnesium-Air Sea Water Primary Batteries," Solar Cells, Vol. II (Cleveland: JEC Press Inc., 1979). This publication shows a "life-torch" with a series-connected twin cell battery of "inside-out" construction, namely a pair of spaced-apart magnesium anodes having a pair of cathodes interposed between them and mutually defining a common air space. Each anode-cathode pair is surrounded by a separate electrolyte space (within a housing) to prevent or minimize electrolytic shunting between the battery cells. As those skilled in the art can appreciate, since the anodes of a pair of series-connected metal/air battery cells are at different potentials, the existence of a current path through the electrolyte between the anodes of the respective cells will cause undesired shunting of current and can significantly impair cell efficiency.
The above-cited publication contemplates use of the described device at sea, attached to a life jacket so that the battery floats substantially immersed in sea water, which enters inlets formed in the housing, one for each cell, separately filling each of the two electrolyte spaces. These inlets are widely spaced apart to reduce electrolytic shunting through the ambient sea. Such a battery uses the sea as the saline electrolyte for the battery and isolates this electrolyte into two separate tanks, one for each battery cell. Thus, to activate the described battery, one need only insert the lantern into the sea.
On land, utilization of a battery constructed in accordance with the above-cited publication would require pouring saline electrolyte into each of the battery inlets. As one can appreciate, the pouring of electrolyte into separate inlet ports can be extremely difficult, especially in the dark. An easier method of filling electrolyte into the batteries is desirable for land applications. Moreover, the device of the above-cited publication is evidently designed for a single use in a marine emergency; for a routine consumer land application, such as during power failure emergencies or extremely inclement weather, it would be desirable to have a battery that could be repeatedly activated by pouring electrolyte into the cells, and repeatedly deactivated by removing the electrolyte from the cells and cleaning out reaction products formed within the cells, without the hindrance of separate tanks for the two cells.
Also, it would be desirable to retard the accumulation of reaction product in the anode-cathode gap of a metal/air cell or battery, such as an aluminum/air battery, thereby to prolong the period of active use of the cell or battery between cleanings. In this regard, it has heretofore been proposed to provide a relatively wide anode-cathode gap for preventing flow of fresh electrolyte around the gap edges, generally parallel to the electrode surfaces; but cell efficiency decreases with increasing anode-cathode distances. Another proposal, set forth in the Handbook of Batteries and Fuel Cells (McGraw-Hill, 1984), p. 30-11, is to prevent hydroxide gel formation by employing a caustic electrolyte rather than a neutral saline electrolyte, but caustic electrolytes are disadvantageous (as compared to saline electrolyte) from the standpoint of convenience, cost, and safety in handling.